widget: make Icon honour its constraints

This is a breaking change as Icon.Layout no longer requests a size.

Before:
  sz := unit.Dp(20)
  ic.Layout(gtx, sz)

After:
  sz := gtx.Metric.Px(unit.Dp(20))
  gtx.Constraints.Min = image.Pt(sz, 0)
  ic.Layout(gtx)

Fixes gio#240

Signed-off-by: pierre <pierre.curto@gmail.com>

widget/icon.go

@@ -7,12 +7,14 @@ import (
 	"image/color"
 	"image/draw"
 
-	"golang.org/x/exp/shiny/iconvg"
-
 	"gioui.org/internal/f32color"
 	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/op/clip"
 	"gioui.org/op/paint"
 	"gioui.org/unit"
+
+	"golang.org/x/exp/shiny/iconvg"
 )
 
 type Icon struct {
@@ -24,6 +26,8 @@ type Icon struct {
 	imgColor color.NRGBA
 }
 
+var defaultIconSize = unit.Dp(24)
+
 // NewIcon returns a new Icon from IconVG data.
 func NewIcon(data []byte) (*Icon, error) {
 	_, err := iconvg.DecodeMetadata(data)
@@ -33,8 +37,17 @@ func NewIcon(data []byte) (*Icon, error) {
 	return &Icon{src: data, Color: color.NRGBA{A: 0xff}}, nil
 }
 
-func (ic *Icon) Layout(gtx layout.Context, sz unit.Value) layout.Dimensions {
+// Layout displays the icon with its size set to the X minimum constraint.
-	ico := ic.image(gtx.Px(sz))
+func (ic *Icon) Layout(gtx layout.Context) layout.Dimensions {
+	sz := gtx.Constraints.Min.X
+	if sz == 0 {
+		sz = gtx.Metric.Px(defaultIconSize)
+	}
+	size := gtx.Constraints.Constrain(image.Pt(sz, sz))
+	defer op.Save(gtx.Ops).Load()
+	clip.Rect{Max: size}.Add(gtx.Ops)
+
+	ico := ic.image(size.X)
 	ico.Add(gtx.Ops)
 	paint.PaintOp{}.Add(gtx.Ops)
 	return layout.Dimensions{

widget/icon_test.go

@@ -9,7 +9,7 @@ import (
 
 	"gioui.org/layout"
 	"gioui.org/op"
-	"gioui.org/unit"
+
 	"golang.org/x/exp/shiny/materialdesign/icons"
 )
 
@@ -26,5 +26,49 @@ func TestIcon_Alpha(t *testing.T) {
 		Constraints: layout.Exact(image.Pt(100, 100)),
 	}
 
-	_ = icon.Layout(gtx, unit.Sp(18))
+	_ = icon.Layout(gtx)
+}
+
+// TestWidgetConstraints tests that widgets returns dimensions within their constraints.
+func TestWidgetConstraints(t *testing.T) {
+	_cs := func(v ...layout.Constraints) []layout.Constraints { return v }
+	for _, tc := range []struct {
+		label       string
+		widget      layout.Widget
+		constraints []layout.Constraints
+	}{
+		{
+			label: "Icon",
+			widget: func(gtx layout.Context) layout.Dimensions {
+				ic, _ := NewIcon(icons.ToggleCheckBox)
+				return ic.Layout(gtx)
+			},
+			constraints: _cs(
+				layout.Constraints{
+					Min: image.Pt(20, 0),
+					Max: image.Pt(100, 100),
+				},
+				layout.Constraints{
+					Max: image.Pt(100, 100),
+				},
+			),
+		},
+	} {
+		t.Run(tc.label, func(t *testing.T) {
+			for _, cs := range tc.constraints {
+				gtx := layout.Context{
+					Constraints: cs,
+					Ops:         new(op.Ops),
+				}
+				dims := tc.widget(gtx)
+				csr := image.Rectangle{
+					Min: cs.Min,
+					Max: cs.Max,
+				}
+				if !dims.Size.In(csr) {
+					t.Errorf("dims size %v not within constraints %v", dims.Size, csr)
+				}
+			}
+		})
+	}
 }

widget/material/button.go

@@ -170,7 +170,8 @@ func (b IconButtonStyle) Layout(gtx layout.Context) layout.Dimensions {
 				size := gtx.Px(b.Size)
 				if b.Icon != nil {
 					b.Icon.Color = b.Color
-					b.Icon.Layout(gtx, unit.Px(float32(size)))
+					gtx.Constraints.Min = image.Point{X: size}
+					b.Icon.Layout(gtx)
 				}
 				return layout.Dimensions{
 					Size: image.Point{X: size, Y: size},

widget/material/checkable.go

@@ -67,7 +67,8 @@ func (c *checkable) layout(gtx layout.Context, checked, hovered bool) layout.Dim
 						if gtx.Queue == nil {
 							icon.Color = f32color.Disabled(icon.Color)
 						}
-						icon.Layout(gtx, unit.Px(float32(size)))
+						gtx.Constraints.Min = image.Point{X: size}
+						icon.Layout(gtx)
 						return layout.Dimensions{
 							Size: image.Point{X: size, Y: size},
 						}